CO2 injection for enhanced oil recovery (EOR) and geological carbon sequestration (GCS) in shale reservoirs offers a promising win-win strategy. Yet, the mechanisms by which CO2 flooding and huff-and-puff (HNP) mobilize multicomponent shale oil in inorganic nanopores remain unclear, particularly regarding their respective contributions to oil recovery and CO2 storage. Here, we employ molecular dynamics (MD) simulations to compare CO2 flooding and HNP in calcite dead-end nanopores saturated with a multicomponent hydrocarbon mixture. We analyze interfacial and transport behaviors, extract localized molecular clusters to quantify self-diffusion coefficients, and examine the roles of flood and flowback pressures. Results show that flooding promotes greater CO2 penetration into nanopores but suppresses counter-current hydrocarbon migration, limiting swelling-driven recovery. In contrast, HNP enables CO2 diffusion during the huff stage and reverse pressure gradients during the puff stage, which enhance oil swelling and achieve higher recovery at the expense of lower storage efficiency. Elevated CO2 density increases the diffusion coefficients of all components, underscoring the critical roles of pressure gradients and molecular transport. By extracting localized molecular clusters, we further obtained pressure-dependent diffusion bounds for CO2 and hydrocarbons and quantify injection and flowback pressure windows that jointly optimize oil recovery and the CO2 storage efficiency. On this basis, we propose a well-count-agnostic and field translatable hybrid method that achieves 60.44% oil recovery and 47.18% CO2 storage efficiency. These findings provide molecular-scale guidance for full-process CCUS design in unconventional reservoirs.
Guo et al. (Wed,) studied this question.